Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics. Such compositions are generally provided as imageable layers.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
A large variety of offset lithographic printing plates comprise a grained anodized aluminum-containing substrate that is coated with a radiation sensitive layer. The radiation is used to either solubilize background areas, leaving relatively soluble image areas, or to insolubilize image areas leaving soluble background areas. The soluble background areas may be removed using a processing liquid (developer) that is generally water-based having a pH of 12 or more. The resulting plates then may be washed with water and treated with a gum solution either for storage or immediate printing. Digital printing plates that are imaged using a laser (either infrared or violet) are mainly developed in a manner similar to non-digital (i.e. analog plates).
The difference in solubility of the background and the image within the developer should be as large as possible so that development is tolerant of varying conditions. However tolerant it may be, there are generally possibilities of under-development when not all of the background coating has been removed and this may cause background scumming during printing, and over-development when fine image areas may disappear. Indeed, for many types of imaged plates, if they are left in the developer indefinitely, the entire image will disappear. Development can be halted when the printing plate is washed so that its surface has a neutral environment and it is further stabilized on the printing machine where the fountain solution is invariably acidic, usually with a pH of from 4 to 6.
In order to optimize the time taken to prepare offset lithographic plates, it is necessary to minimize the time for imaging exposure. Where the process is one of cross-linking or polymerization of the material using for instance a laser beam, the time can be minimized by using just enough energy to permit the background to be dissolved by the processing solution. However, in choosing to minimize imaging time the extent of cross-linking/polymerization may be limited and thus incomplete. If this is the case, then the run length of the plate may be curtailed by image erosion. Thus there can be a conflict among imaging speed, processing conditions, and plate run length. This problem may be minimized by treating the imaged printing plate with heating before processing, or even by heating the printing plate precursor prior to imaging to improve plate sensitivity. However, the use of a heating step requires extra time and energy and is a disadvantage.
Most negative-working imageable elements are processed using high pH processing solutions that may deteriorate both from use, as the solution of plate material gradually lowers the pH, and from the absorption of carbon dioxide from the air which has a similar effect. This problem is ameliorated by providing a replenishing liquid that is introduced into the processor to maintain the potency of the developer. High pH liquid developers cannot be disposed of down the drain where prohibited by law.
Recently, the industry has produced printing plates that can be developed on-press either by means of the fountain solution or the lithographic ink, or by a combination of both. This has certain advantages. The customer dispenses with all of the problems mentioned above relating to the developer as well a gaining from being able to operate without a large processor unit. However, because development is done during printing conditions, development tends to continue so long as there is something to develop. In order to avoid this as much as possible the plates developed have been negative-working. Even if the image is gradually developed away during a long print run by using a polymerized image, optimal run length has been achieved. However although images may be obtained at a lower laser exposure, such images can be easily worn away. Thus it is beneficial to work at relatively high laser exposures in order to achieve optimum print run. This means that in practice, lithographic printing plates developed on-press plates are not of the highest sensitivity.
A further problem in the design of on-press developed printing plates is that the non-imaged material that is removed from the printing plate must not be so colored or the color will find its way into the ink and contaminate the image. This may be especially evident in the yellow ink. The corollary of this is that the image/background on imaged printing plate before it is placed on the printing press has minimal coloration contrast. It could be unreadable or may not be used to calibrate the CTP (computer to plate) imaging system to be certain of working at maximum setting.
Despite all of these problems, customers still find the advantages of on-press processing to be attractive.
A second approach to coping with the problems of separate development is to try to simplify the processing so that the processor is more compact and the processing liquid is stable and can be disposed of in the sewerage. It is advantageous in these respects if the developer is neutral or acidic, but if this is the case then there is a danger that the plate will continue to develop in the presence of the acidic fountain solution on the press.
US 2008/0199804 (Oohashi) describes some of the problems of simple and on-press processing that are described above. This publication describes printing plate precursor formulations containing radical polymerizable compositions having high sensitivity and being capable of development at pH of 10 or less on a printing press. After development, the plate may be floodwise exposed using various types of radiation. But as described in the publication, if the radiation is such that it would be affected by surrounding light under a working environment, it is preferable that the plate-making line is blinded by a cover. It is unclear from this publication as to the pH of the aqueous developer (the preferred development is from 11.5 to 12.5). Thus Oohashi fails to appreciate the advantages of acid processing and fails to produce printing plates that could overcome the disadvantages of using high pH development.
We have discovered a need to provide low pH processing while maintaining high sensitivity and long run length in lithographic printing plates obtained from negative-working imageable elements.